My bank building is at level 1, meaning I can only store a maximum of 1500 gold. How do I upgrade this building? When I click on it, there is no upgrade button. I seem to be in a bit of a deadlock as I can't figure out anything to buy, since all of my remaining building upgrades cost more than 1500 gold, and I can't figure out how to trigger a bank upgrade.

The bank and Bezar upgrades come in alternating order. If you have an outstanding Bezar quest, you need to do that before the next Bank event can trigger. The third bank upgrade requires a trip to the Shifting Passages, which is a dungeon to the far north, and the third Bezar quest requires a trip to Hexx Ruins, which is to the south. You won't get the prompts for these quests until you have the appropriate dungeons on your map. I think that the 3rd Bezar quest also requires you to have access to Ick Swamp, even though the quest itself occurs in Hexx.